Four decades ago, it was thought that the cardiac β-adrenergic receptor (AR) was β1AR, the vascular/bronchial counterpart was β2AR, and that β2AR was either nonexistent or nonfunctional in myocardium.1 In the heart, stimulation of β1AR leads to PKA-dependent phosphorylation of a set of Ca2+ regulatory proteins, including sarcolemmal L-type Ca2+ channels, sarcoplasmic reticulum (SR) Ca2+-release channels (ryanodine receptors), SR Ca2+-ATPase (SERCA) and its regulator phospholamban (PLB), and some myofilament proteins, resulting in positive inotropic, lusitropic, and chronotropic effects. However, over the past decade, compelling evidence has shown that the β2AR subtype is expressed in the heart and its signaling and functionalities markedly differ from those evoked by the closely related βAR subtype, the β1AR. Unlike β1AR, β2AR couples dually to Gs and Gi proteins; the β2AR-Gi signaling pathway plays a crucial role in cardioprotection against apoptotic death of myocytes in culture and in vivo (the “good”), while attenuating the β2AR-Gs-mediated inotropic response (the “bad”) (Figure).2 Now, in the current issue of Circulation Research, He et al revealed one “ugly” facet of the β2AR-Gi signaling in a canine heart failure model.3 They demonstrated that in the failing heart, activation of β2AR dampens the ability of β1AR, the primary cardiac subtype, to stimulate ICa,L, thus resulting in an overall dysfunction of βAR inotropic response in the failing heart (Figure).3 Specifically, the effect of βAR stimulation with a nonselective agonist, isoproterenol (ISO), on ICa,L is strikingly diminished in cardiomyocytes from canine failing heart, but can be revived by disruption of Gi function with pertussis toxin (PTX) or β2AR blockade with ICI 118 551.3 These findings highlight that an alteration in the status of the β2AR-Gi coupling can dictate the overall outcome of cardiac βAR signaling under some pathological circumstances. Thus, this enigmatic, multifaceted β2AR-Gi signaling pathway might bear important pathogenic and therapeutic implications.

The “Good”: Cardioprotection Induced by Sustained β2AR Stimulation

A large body of evidence gleaned from pharmacological and mouse genetic studies has revealed opposing contributions of sustained β1AR and β2AR stimulation in regulating the fate of cardiomyocytes. Whereas sustained β1AR stimulation promotes apoptotic death of cardiomyocytes, sustained stimulation of β2AR protects myocytes against a wide range of apoptotic insults. For instance, agonist-induced β2AR stimulation prevents catecholamine-, hypoxia-, or reactive oxygen species (ROS)-induced apoptotic death in both neonatal and adult rat cardiomyocytes.4–6 Moreover, in adult mice lacking the native β2AR, stimulation of the native β1AR by catecholamine causes overtly exaggerated cardiomyopathy, myocyte apoptosis, and more severe heart failure relative to wild-type control animals.7 In contrast, selective activation of β2AR by fenoterol for 8 weeks exerts a clear antiapoptotic effect and improves cardiac performance in a myocardial infarction–induced rat heart failure model.8 These in vivo studies have provided evidence that β2AR stimulation exerts a cardiac protective effect in response to elevated circulating catecholamine levels or myocardial infarction.

The cardiac protective effect of persistent β2AR signaling is largely mediated by β2AR-Gi coupling, which, in turn, activates a cell survival pathway sequentially involving Giβγ, PI3K, and Akt. First, β2AR blockade enhances β1AR-induced apoptosis in cultured adult rat myocytes in a PTX-sensitive manner, suggesting the β2AR protective effect is Gi-dependent.4 Second, β2AR, but not β1AR, activates a Gi-Gβγ-PI3K-Akt cell survival signaling pathway6,9, and inhibition of this pathway abolishes the ability of β2AR to block hypoxia- and ROS-induced myocyte apoptosis.6 Thus, the β2AR-Gi- Gβγ-PI3K-Akt signaling cascade not only counteracts βAR-induced apoptosis and but also protects cardiomyocytes against other apoptotic stimuli.

The “ Bad” or “Ugly”: β2AR-coupled Gi Negates β1AR- and β2AR-Mediated Contractile Support in the Failing Heart

Although beneficial in terms of cardiac protection, the β2AR protective effect comes at the cost of compromised contractile support. Previous studies have demonstrated that the β2AR-Gi functionally restricts the β2AR-Gs–mediated cAMP/PKA signaling to subsarcolemmal microdomain in the vicinity of L-type Ca2+ channels, thus preventing the Gs-PKA mediated phosphorylation of some key target proteins in SR membrane and intracellular contractile myofilaments, blunting the positive inotropic and lusitropic effects.10–13 Activation of PI3K, an important downstream event of the β2AR-Gi signaling, confines and minimizes the concurrent β2AR-Gs–evoked cAMP/PKA signaling.14 In the failing heart, an upregulation of Gi15 and a selective downregulation of β1AR16 are often associated with enhanced β2AR-Gi signaling and reduced myocardial contractile response to both β1AR and β2AR stimulation. Importantly, inhibition of the Gi signaling pathway with PTX restores the diminished βAR inotropic response in a variety of heart failure models, including a spontaneous hypertensive rat heart failure model,17 a myocardial infarction rat heart failure model,18 and myocytes from failing human hearts.19 Furthermore, in failing porcine and mouse hearts or cardiomyocytes, inhibition of βAR-targeted PI3K, the major downstream mediator the Gi signaling, improves the contractile function of the failing myocardium.20,21 Now, He and colleagues demonstrate a cross-inhibition of β1AR-mediated stimulation of ICa,L by the β2AR-Gi signaling.3 Similarly, the β2AR-Gi signaling largely inhibits β1AR-induced positive inotropic effect in adult rat cardiomyocytes moderately overexpressing Na+/Ca2+ exchanger proteins.22 Collectively, these studies suggest that reinforcement of β2AR-Gi signaling is a hallmark of the failing heart and is critically involved in heart failure–associated dysfunction or desensitization of both βAR subtypes.

The Cell Logic of Multifaceted β2AR-Gi Signaling

At the first glance, inhibition of the β1AR-mediated stimulation of ICa,L and, consequentially, the contractile response by β2AR-coupled Gi might paint β2AR stimulation as the “bad guy” in the context of heart failure. It is, however, noteworthy that sustained β1AR stimulation induces myocyte apoptosis and positive inotropic effect mainly via PKA-independent stimulation of L-type Ca2+ channels and resultant activation of CaMKII in adult mouse and rat cardiomyocytes (Figure).23,24 Inhibition of ICa,L or CaMKII can effectively protect the cultured cardiomyocytes from β1AR-induced apoptotic death.23 In contrast, overexpression of the L-type channel (α1C) causes severe cardiac hypertrophy and apoptosis.25 Recent in vivo studies have further confirmed that inhibition of CaMKII substantially prevents cardiac maladaptive remodeling from excessive βAR stimulation and myocardial infarction and markedly improves cardiac function (Figure).26 In light of these observations, we envision that the inhibitory effect of the β2AR-Gi signaling on β1AR-mediated activation of ICa,L and resultant CaMKII may represent an intrinsic cardiac protective mechanism, acting as a “friend” rather than a “foe,” to protect the heart against apoptosis and maladaptive remodeling in response to chronic catecholamine stimulation. Thus, the apparent “bad” or “ugly” behavior might be an overreaction of the defense mechanism; appropriately tipping the balance might be able to bring out the “good” nature of β2AR-Gi signaling to benefit the struggling heart.

The exact mechanism underlying the cross-inhibition of β1AR function by the β2AR-Gi signaling remains elusive. There are several candidate mechanisms, including the β2AR-Gi signaling–mediated direct suppression of adenylyl cyclase activity or activation of PI3K. With respect to the latter, it has been shown that activation of PI3K inhibits ICa,L in normal adult rat cardiomyocytes.27 More importantly, inhibition of membrane-targeted PI3K activity ameliorates cardiac dysfunction and improves survival in multiple heart failure models.20,21

Alternatively, we have recently demonstrated that β1AR and β2AR are able to form heterodimers in adult mouse cardiomyocytes and HEK 293 cells.28,29 Specifically, in cardiomyocytes, the heterodimeric receptors exhibit altered ligand binding profiles, enhanced signaling efficiency in regulating myocyte cAMP production and contractility, and suppressed β2AR spontaneous activity in the absence of agonist stimulation, thus optimizing β-adrenergic regulation of cardiac contractility (Figure).28 Interestingly, heterodimerization between β1AR and β2AR inhibits the agonist-promoted internalization of β2AR and its ability to activate the Gi-ERK1/2 MAPK signaling pathway in HEK 293 cells.29 Similarly, whereas either β2AR or β3AR alone couples to both Gs and Gi proteins, the β2AR-β3AR heterodimer is unable to activate Gi signaling.30 Thus, alterations in the status of oligomerization of GPCRs from the same or different families may lead to changes in the selectivity and specificity of G protein coupling of those receptors, thereby altering their signaling and functional features, perhaps also raising important therapeutic considerations.

The heart failure–associated decrease in the ratio of β1AR to β2AR,3,16 in conjunction with changes in cardiomyocyte morphology and membrane integrity, might interfere with the heterodimerization of the remaining βARs, thus allowing the β2AR to better couple to Gi proteins. The enhanced Gi signaling inhibits β1AR-mediated increases in ICa,L and contractility, perhaps most importantly, ameliorates β1AR-evoked maladaptive remodeling and loss of cardiomyocytes (Figure). These hypotheses merit future investigation.

In summary, it is reasonable to speculate that the selective downregulation of β1AR and the upregulation of β2AR-coupled Gi signaling in the functionally compensated hypertrophied heart may represent salutary cardiac adaptation, which may protect myocytes against apoptosis and maladaptive remodeling and consequently slow the progression of cardiomyopathy and contractile dysfunction. However, exaggerated β2AR-Gi signaling blunts the Gs-mediated stimulation of ICa,L and contractile support, thus contributing to the contractile defect of the failing heart despite of its antiapoptotic effect. Thus, restoration of the Yin and Yang balance of β2AR-coupled Gi and Gs signaling cascades may open a novel therapeutic avenue for the treatment of heart failure.

Acknowledgments

This work is supported by National Institutes of Health intramural research grant (to Z.W.Z., X.Z., and R.P.X.), and in part by Chinese National Key Project 973 (G2000056906) and Chinese Young Investigator Award (30225036). The authors thank Dr H. Cheng for critical comments and discussions.

Footnotes

The opinions expressed in this editorial are not necessarily those of the editors or of the American Heart Association.